GEOSYNTHETICS ENGINEERING: IN THEORY AND … 48.pdf · GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE ... soils which are highly plastic and swelling in nature. ... Geofoam The

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay


Module - 9LECTURE - 48

Geosynthetics for ground improvement

Recap of previous lecture…..


Excel program for PVD drains

Geosynthetic encased stone columns

Encasements

Axisymmetric finite element model (fem) of encased stone column

Finite Element Modelling of Encased Stone Columns

Axisymmetric finite element model (FEM) of encased stone column (After Dutta et al., 2012)


FEM fine mesh generation

- 15-node triangular elements tomodel the deformations andstresses in the soil

- Un-drained (B) Mohr-Coulombmodel for soft clay

- Drained Mohr-Coulomb modelfor stone column.

- The geogrid elements aremodeled as elastic material.

- Short term plastic analysis ascalculation procedure.


MATERIAL PROPERTIESProperties of clay and stone:

Parameters PropertiesClay Stone

Elastic modulus, E' (kPa) 4000 50000Poisson’s ratio, ν' 0.4 0.3Cohesion, cu (kPa) 10 0

Angle of internal friction, φ 0 45°

Properties of geogrid: Length of geogrid: 2D, 4D and full length (5D)

D = diameter of column

Stiffness of geogrid: 200 kN/m, 300 kN/m, 1000 kN/m and 2000 kN/m


The analysis has been carried out keeping in view:

- Radial deformation of stone column without and with encasement,

- Relative shear stress distribution,

- Pressure - settlement response, • Effect of length, and• Effect of stiffness of encasements


Radial Deformation:

- A huge radial displacement ofabout 13 mm in case of ordinarystone column (OSC)

- Full length encasement of stiffness300 kN/m reduce the maximumradial displacement to 4 mm.

Radial deformation (a) ordinary stone column (b) encased stone column (300 kN/m)


Effect of encasement stiffness on radial deformation Full length Encasements

Stiffness 1000 kN/m maximum radial deformation = 2 mm

Stiffness 2000 kN/mmaximum radial deformation = 1.4 mm

- More hoop tension force getsdeveloped in the stiffer encasement atthe same applied load resulting moreconfining pressure to the stonecolumn.

- For encasement stiffness of 200kN/m, 300 kN/m, 1000 kN/m and2000 kN/m, hoop tension is 18.13kN/m, 23.81 kN/m, 44.49 kN/m and56.21 kN/m respectively.

Radial deformation of encased stone column with stiffness (a) 1000 kN/m and (b) 2000 kN/m


Relative Shear Stress Distribution

Relative shear stress distribution (a) ordinary stone column (b) encased stone column (2000 kN/m)

- Relative shear stress is defined as the ratio of mobilizedshear stress to the maximum shear stress.


Influence of stiffness of encasement

Pressure - settlement response of stone column without and with encasement of different stiffness values


Influence of length of encasement

Pressure - settlement response of encased stone column with different

encasement lengths

For the same prescribeddisplacement,

the maximum hoop tensiondeveloped in the partialencasements of 2D and 4Dlength are 7.462 kN/m and44.64 kN/m respectively

while the maximum hooptension is 56.21 kN/m in the fulllength encasement


Influence of stiffness on length of encasements

Influence of encasement stiffness on its various lengths


Encasing the stone column with suitable full lengthencasement can increase the bearing capacity of claymany times than that of the ordinary stone column.

Encasement of higher stiffness provides more confiningpressure to the stone column due to the generation ofhigher hoop tension force in it.

When the stone column is encased, the applied load isdistributed to entire length of the column whereas theordinary stone column fails due to the lateral bulging ofthe stones within 2D length of the column from the top.

CONCLUSIONS


The mobilized shear stress zone is more in encasedcolumn when compared with ordinary stone column forthe prescribed settlement.

For an encasement with certain stiffness, the loadcarrying capacity gets improved with the increase inlength of the encasement.

The effect of encasement stiffness is negligible whenthe encasement length is very short. However, as thelength of encasement is increased, the stiffer encasementproduces better results and it is more pronounced for fulllength of encasements.


Analytical Study on Encased Stone Column

Uniform lateral deformation of stone column (simplified assumption) (Wu and Hong, 2009)

It can be assumed that the load is transferred uniformlycausing a uniform lateral deformation along the entirelength of the column.

For more simplicity, it can also be assumed that thevolume always remains constant while the uniform lateraldeformation takes place.

Ri = initial radius of the columnHi = initial length of the columnRf = final radius of the columnafter deformationHf = final length of the column


i

fil H

HH

1

fi 1

HH

fHRHR 2

fi2

i

li

f

l

f

if

iif 1

1RH

1H

RHHRR

l

l

i

il

i

i

if

i

ifr

111

R

R1

1R

RRR

R2)RR(2

If axial strain of the fully encased stone column = εl

If volume is constant at the time of deformation,

Therefore,

At any stage of loading, if circumferential strain or hoofstrain = εr

- For the calculated hoop strain, tensilestrength of geogrid can be determinedfrom tensile strength versus strain curveobtained from tensile strength test.


Considering the encasement as a thin cylindricalelement, generation of circumferential hoop stress in theencasement is shown below.

Hoop stress generation in a cylindrical geogrid encasement

If confining pressure = Pc and hoop stress = Ph,

LxDxP2xLxtxP fch tT

t2DxPP fc

h

Df = final diameter of the stonecolumnL = length of the encasementT = tensile strength of the geogridper unit length at the correspondinghoop straint = thickness of the thin cylindricalencasement


GROUND IMPROVEMENT USING GEOCELL

In India, half of the subcontinents consist of black cottonsoils which are highly plastic and swelling in nature.

It is a very serious problem for the engineers toconstruct embankments or reinforced soil retaining wallson swelling soils.

The conventional methods are excavation andreplacement with good quality filling materials or pilingwhich is not economical and practical.

Alternatively, geocell mattress can be used as aneffective ground improvement technique.


What is Geocell?

Geocell is a three dimensional honeycomb structuresmade of a series of interlocking cells.

Jute geocell mattress Uniaxial geogrid Geocell mattress


Geocell may be made of Geogrid Non-woven and woven geotextile Plastic and Geofoam

The geometry of cell may be triangular, square,rectangular and hexagonal. It provides very goodconfining effects.

The deployment of geocell reinforced mattressensure stiff platform and drastically improve the bearingcapacity of weak soil.


Mhaiskar and Mandal (1996) conducted plate load test onsoft saturated marine clay subgrade to find out the efficiencyof geocell, effect of its cell geometry as well as effect ofrelative density of the backfill material.

They reported considerable improvement in load carryingcapacity as well as reduction in settlement when the clay isreinforced with geocell.

It was found that the ultimate bearing capacity as well asbearing capacity ratio of marine clay increases withdecreasing width to height ratio of geocell.

A three-dimensional finite element analysis was alsocarried out using 'ANSYS' to validate the experimentalresults. The finite element results were found in accord withthe experimental results.


Reinforcement Mechanism- Unlike the unreinforced base, geocell-reinforced basecan provide lateral and vertical confinement, tensionedmembrane effect, and wider stress distribution.

- As the geocell is a three dimensional structure, it canprovide lateral confinement to soil particles within cells.Geocell mattress can provide the lateral and verticalconfinement in following ways: Lateral confinement:

(1) As the load over the mattress increases, hoop stress is generatedin the cell wall resulting in the lateral confinement to the infill material.

(2) Lateral confinement from the adjacent cells to prevent the lateralexpansion of any cell.


Vertical confinement:(1) The friction between the infill material and the geocellwall, and

(2) The geocell-reinforced base acts as a mattress to restrainthe soil from moving upward outside the loading area.

Unreinforced and geocell-reinforced bases (After Pokharel et al., 2010)


Design ConsiderationsThe reinforcing effect can be increased by increasingthe height of cell, width of mattress and decreasing theequivalent diameter of cell.

Effect of height Effect of width

Effect of equivalent diameter of the single cells

Effect of tensile stiffness

Effect of infill soil Depth of embedment

Effect of basal reinforcement


Area of applications:

Under pavements

Beneath the embankments

Over column treated grounds

Over buried pipes

Landfill


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


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GEOSYNTHETICS ENGINEERING: IN THEORY AND … 48.pdf · GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE ... soils which are highly plastic and swelling in nature. ... Geofoam The